1. Field of the Invention
The present invention relates generally to geophysical exploration and more particularly to methods for accurately estimating uncertainties in pore pressure and fracture gradient estimation prior to drilling of a well.
2. Description of the Related Art
Drilling of wells is carried out using a bottomhole assembly that includes a drillbit. During the drilling process, drilling fluid, also referred to as drilling mud, is pumped down the borehole to facilitate the drilling process, cool and lubricate the drillbit, and remove drill cuttings to the surface. If the borehole fluid pressure is significantly below the formation fluid pressure, there is a risk of a catastrophic blowout. On the other hand, if the borehole pressure is much greater than the formation fluid pressure, the risk of blowout is eliminated but there is risk of formation damage due to fracturing and the mud invading the formation. The fluid pressure is a function of the density of the drilling mud (“mud weight”) so an important part of the drilling process is the proper selection of mud weight for drilling.
It is standard practice when planning wells to utilize seismic data to compute pore pressure and fracture gradient profiles to use as upper and lower bounds on required mud weights for safe drilling. U.S. Pat. No. 6,473,696 to Onyia et al. discloses a method of determination of fluid pressures in a subsurface region of the earth that uses seismic velocities and calibrations relating the seismic velocities to the effective stress on the subsurface sediments. The seismic velocities may be keyed to defined seismic horizons and may be obtained from many methods, including velocity spectra, post-stack inversion, pre-stack inversion, VSP or tomography. Overburden stresses may be obtained from density logs, relations between density and velocity, or front inversion of potential fields data. The various methods are part of an integrated computer program.
Sayers et al. disclose a method for the use of seismic velocities used during seismic processing to optimize the stack/migration result, with local fluctuations being smoothed out and the velocity sampling interval usually being too coarse for accurate pore pressure prediction. Various methods of determining seismic interval velocities from prestack seismic data are compared, and a velocity analysis approach suitable for pore pressure prediction is recommended.
Methods have also been developed for identification of shallow water flow hazards where abnormally high pore pressures exist in shallow sub-bottom sediments drilled in deep water. U.S. Pat. No. 6,694,261 to Huffman teaches the detection of such abnormally pressured zones by amplitude versus offset (AVO) analysis of the reflected amplitudes of compressional or shear reflections. Measurements of the amplitude of reflected shear waves from a formation at some depth below the anomalous zone may also be used to detect the presence of abnormally pressured intervals with low shear velocity and high shear wave attenuation. US 2003/0110018 of Dutta et al. addresses the identification of shallow water flow hazards using seismic inversion methods.
None of the methods discussed above address the issue of errors caused by uncertainty in the measurements and by uncertainty in the modeling process. By quantifying uncertainties in pore pressure and other predicted values, and more importantly by determining their origin, it is possible not only to begin to quantify the drilling risk but also to make decisions about how best to reduce that risk. For example, if uncertainties in the velocities used as input to the predictions contribute large uncertainties to the results, this may dictate reanalysis of the seismic data. If uncertainties are related to the functions used to compute density or effective stress, this might lead to a recommendation to reduce those uncertainties using additional measurements on core or using offset log data.
Liang discloses application of a method of quantitative risk analysis (QRA) to the problem of pore pressure and fracture gradient prediction. The method relies on a vast sampling over a prospect area of borehole measurements to determine such parameters as density, acoustic velocity and pressure gradient. The uncertainties are then determined from variations in the measured parameters. Underlying this uncertainty determination is the assumption that measurements of parameters such as density and acoustic velocity are invariant with spatial location (“the ground truth”), and the variations are inherent. This is not a reasonable assumption as it is well known that there are systematic variations in velocity and density with spatial location. In addition, Liang assumes an Gaussian distribution to characterize the undertainty. Such an assumption is commonly not satisfied, and distributions like the log-normal are quite common. Furthermore, Liang does not account for overpressure mechanisms other than undercompaction. It would be desirable to have a method of QRA that is applicable to the problem of pore pressure and fracture gradient prediction that does not make these assumptions and does not require a large sampling of measurements to establish the ground truth. The present invention satisfies this need.